This section intends to provide a background or context for the embodiments of the present disclosure described in the claims. Although being included in this section, the descriptions herein will not be deemed as the prior art.
The technology of Image Guide Radiation Therapy (IGRT) is the most important means in the precise radiotherapy at present. The IGRT can provide exact position information for precise lesion localization and tumor irradiation, and has been widely used in the modern minimally invasive surgery and radiotherapy. The X-ray CBCT installed beside the treatment couch is an important means for implementing image guide nowadays.
However, in accompany with the expansion of irradiation volume in a single projection, the quality of the cone-beam computed tomography (CBCT) image will be rapidly deteriorated due to the restriction of a fundamental physical process—scattering pollution. The traditional CT reconstruction theory assumes that the X-ray is in a rectilinear propagation, and the detected ray intensity attenuates with the integral index. The scattered photons deviate from the incident beam direction and cannot be modelled into the traditional CT reconstruction theory, which is the error source of the CT image reconstruction. Studies show that the strength of the scattering signal monotonically increase while the irradiation volume of the X-ray expands. The CBCT scattering pollution seriously affects the precision of the CT value, the detectability of the target with a low contrast, and the accuracy of the dose calculation, and those disadvantages directly disable the CBCT for wide clinical applications. When a human body is scanned in a CBCT system having no scattering correction, the CT value error caused by the scattering artifact can reach 350 HU, thus the CBCT is mainly used for the primary localization and positioning, and its further applications in the intervention and radiotherapy are seriously restricted. Therefore the scattering correction is the problem to be firstly solved for improving the quality of the CBCT image.
The currently known scattering correction methods mainly include two types, i.e., the pre-processing method and the post-processing method. The pre-processing method performs a scattering correction mainly by attaching a hardware device to prevent the scattered photons from arriving at the detector, so that the scattering signal and associated statistic noises are suppressed in the projection. Two typical examples of the pre-processing method include expanding the air gap between the detector and the object, and using the anti-scatter grid. With the expansion of the air gap, the detectivity of the diffused scattered photons is decreased, while the source signal will not be influenced. But the method is restricted by the physical space of the CBCT device itself, while requiring adding the X-ray dose to compensate for the increase of the distance, thus it is not practical in clinical practices. The anti-scatter grid uses lead meshes focusing at the ray source and can block the scattered light from the non-focusing incident angle. The method has the defect that the attenuation efficiency of the scattered light is not high. Currently, the commercial grid only provides an SPR reduction rate of about 3 times, and cannot ensure the quality of the CBCT image under the high scattering environment. In addition, it also requires adding the patient's exposure dose to compensate for the intensity of the source ray attenuated.
In view of the limitation of the pre-processing method, the post-processing method is more studied at present. The post-processing means perform a scattering correction after obtaining the projection image of the scattering pollution in the traditional way. Since it is impossible to theoretically predict the random scattering time, the scattering noises still remain in the image even a perfect pre-processing method is used. There are many pre-processing methods, including analytical modeling method, Monte Carlo simulation method, source modulation method and measurement method. The analytical modeling method deems that the scattering signal is a response after the source signal passes through a scattering kernel which is generally obtained through a measurement or simulation. If the scattering kernel has the characteristic of unchanged linear translation space, the calculation speed will be fast. But correspondingly, the scattering estimation accuracy is limited and the parameters shall be tediously adjusted for a complex object. The Monte Carlo simulation method establishes a more accurate statistical model for the scattering signal by simulating the interaction between the photons and the illuminated object. But the method has a huge calculation amount, thus much time is cost, and the current computer computing power restricts its application in the CBCT image reconstruction that almost requires a real time processing. The source modulation method adds a high-frequency modulator between the X-ray source and the object, and separate the scattering signal and the source signal from each other in the frequency domain according to different response characteristics thereof. The method does not increase the patient exposure dose or scanning time, but the clinical application effect is restricted by actual physical factors, such as the spiral arm vibration and big focus size.
A measurement-based scattering correction is an implementation method most similar to the present disclosure. The method adds a beam blocking grating in front of the CBCT ray source to estimate the scattering signal, so that a shadow area only containing the scattering signal is formed on the detector. Since the scattering distribution mainly concerns the low-frequency components and slightly disturbed by the blocking grating, the whole scattering distribution can be obtained by a scattering sampling interpolation for the shadow area of the detector. The measurement-based method can obtain an accurate scattering estimation, but the cost is the loss of the source signal. Thus, people usually need to scan each angle twice (one using the blocking grating, and the other removing the blocking grating), or move the blocking grating in the scanning process.
The measurement-based scattering correction method is possible since the blocker has a low cost and can be easily made. The Chinese patent No. 201410380731.3 proposes a method and apparatus for cone beam CT scattering correction based on complementary grating. The invention performs a scattering correction of a projection image through complementary grating scanning and a small calculation amount, and the scattering correction slice images can be reconstructed using the scattering-corrected projection image. The Chinese patent No. 201010574162.8 proposes a CT system and a scattering correction method for the same. The invention acquires the bright field image, places the blocker between the detector and the object to be scanned, and obtains the attenuation projection image after scanning; next, scans the object to be scanned and the scattering corrector, respectively, to obtain a projection image set and a scattering correction image; next, generates a scattering signal distribution according to the bright field image, the scattering correction image and the attenuation projection image, and finally obtains a corrected projection according to a difference between the projection image set and the scattering signal distribution.
Most of the existed scattering correction methods using the blocker need to scan twice to compensate for the blocked original projection signal, and they cannot be put into the clinical application since the patient's expose dose is increased. In addition, those methods are used in the desktop CBCT experimental platform; since such experimental platform replaces the rotation of the light source and the detector with the rotation of the object to be scanned, the isocenter of the CT system is stable, and the projection positions of the blocker at different timing are almost consistent with each other, thus the difficulty for performing a scattering correction using the blocker is largely decreased, and those methods are only applicable for the lab desktop CBCT system with a stable isocenter. However, in the clinical CBCT system, due to the vibration in the rack rotation process and the deviation of the rotation isocenter, the projection position of the blocking grating varies with the rotation of the rack, and it is difficult to accurately extract the scattering signal in the grating area.